Liquid circulation heating system and method of controlling the same

ABSTRACT

A liquid circulation heating system includes a heat pump for producing a heated liquid and a heating radiator. The heat pump has a heat pump circuit in which a compressor, a radiator, a decompressor, and an evaporator are connected in series. The heat pump circuit is charged with a zeotropic refrigerant mixture of at least two refrigerants having different boiling points. The system is configured so that the composition ratio of a higher boiling point refrigerant in the refrigerant circulating through the heat pump circuit increases when the liquid supplied to the radiator has a relatively high temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid circulation heating system forperforming air-heating using a liquid and to a method of controlling thesystem.

2. Description of Related Art

Conventionally, there has been known a liquid circulation heating systemfor producing hot water by a boiler or an electric heater and performingair-heating using the hot water produced. In recent years, the use of aheat pump capable of producing hot water with high efficiency has beenconsidered as an alternative heat source to a boiler and an electricheater. For example, JP 2008-39306 A proposes a liquid circulationheating system for producing hot water by a heat pump and storing theproduced hot water in a hot water storage tank. In this liquidcirculation heating system, the hot water stored in the hot waterstorage tank is fed to, for example, a heating radiator placed indoorsto radiate its heat, and then returned to the hot water storage tank.

The heat pump has a heat pump circuit for circulating a refrigerant. Theheat pump circuit includes, for example, a compressor, a radiator, anexpansion valve, and an evaporator, which are connected by pipes. Heatis exchanged between a refrigerant and water in the radiator so as toheat the water, and thereby hot water is produced.

SUMMARY OF THE INVENTION

In the liquid circulation heating system, when the flow rate of the hotwater fed to the heating radiator is high, the temperature of the waterthat flows from the heating radiator presumably does not drop so much.In this case, the temperature of the water supplied to the radiator ofthe heat pump rises. When the temperature of the water supplied to theradiator rises, the high pressure of the refrigeration cycle increases,as shown by a dotted line in FIG. 2.

When the high pressure of the refrigeration cycle increases, however,the increased pressure may exceed the upper limit pressure for ensuringthe normal operation of the components of the heat pump.

In view of the above circumstances, it is an object of the presentinvention to provide a liquid circulation heating system capable ofsuppressing an increase in the high pressure of the refrigeration cyclewhen the temperature of the liquid supplied to the refrigerant radiatorrises.

When the temperature of the liquid supplied to the refrigerant radiatorrises, a high boiling point refrigerant causes a smaller increase in thehigh pressure of the refrigeration cycle than a low boiling pointrefrigerant does. Moreover, when a mixture of these high and low boilingpoint refrigerants is used, how much the high pressure of therefrigeration cycle increases as the temperature of the liquid suppliedto the refrigerant radiator rises is determined by the mixture ratiobetween these refrigerants. Accordingly, the inventors of the presentinvention have considered that the increase in the high pressure of therefrigeration cycle can be suppressed by actively taking advantage ofthe phenomenon that when a zeotropic refrigerant mixture of refrigerantshaving different boiling points is used as a refrigerant, thecomposition of the refrigerant circulating through the heat pump circuitchanges. The present invention has been made in view of the abovecircumstances.

The present invention provides a liquid circulation heating system forperforming air-heating by heating a liquid to produce a heated liquidand releasing heat of the heated liquid from a heating radiator. Thissystem includes a heat pump circuit for circulating a refrigerant, andthis heat pump circuit includes a refrigerant radiator for heating theliquid by radiating heat from the refrigerant to produce the heatedliquid. The heat pump circuit is charged with a zeotropic refrigerantmixture of at least two refrigerants having different boiling points, asthe refrigerant. The system is configured so that a composition ratio ofa higher boiling point refrigerant in the refrigerant circulatingthrough the heat pump circuit increases when the liquid supplied to therefrigerant radiator has a relatively high temperature.

The present invention also provides a method of controlling a liquidcirculation heating system for performing air-heating by heating aliquid to produce a heated liquid and releasing heat of the heatedliquid from a heating radiator. The liquid circulation heating systemincludes a heat pump circuit for circulating a refrigerant, and the heatpump circuit includes: a refrigerant radiator for heating the liquid byradiating heat from the refrigerant to produce the heated liquid; acompressor for compressing the refrigerant; a decompressor fordecompressing the refrigerant; an evaporator for evaporating therefrigerant; and a vapor-liquid separator for separating the refrigerantinto a gas refrigerant and a liquid refrigerant. The heat pump circuitis charged with a zeotropic refrigerant mixture of at least tworefrigerants having different boiling points, as the refrigerant. Thismethod includes the step of controlling an operation of the decompressorto reduce an amount of the liquid refrigerant in the vapor-liquidseparator when the liquid supplied to the refrigerant radiator has arelatively high temperature.

The present invention makes it possible to suppress the increase in thehigh pressure of the refrigeration cycle when the temperature of theliquid supplied to the refrigerant radiator rises.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a liquid circulationheating system according to a first embodiment of the present invention.

FIG. 2 is a Mollier diagram of a heat pump.

FIG. 3 is a diagram for explaining the fact that an increase in the highpressure of the refrigeration cycle is suppressed due to a change in thecomposition of the refrigerant circulating through the heat pumpcircuit.

FIG. 4 is a schematic configuration diagram of a liquid circulationheating system according to a second embodiment of the presentinvention.

FIG. 5 is a schematic configuration diagram of a liquid circulationheating system according to a third embodiment of the present invention.

FIG. 6 is a schematic configuration diagram of a heat pump of a firstmodification.

FIG. 7 is a schematic configuration diagram of a heat pump of a secondmodification.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. It should be noted, however,that the embodiments described below are merely examples of the presentinvention, and should not be construed to limit the scope of the presentinvention.

First Embodiment

FIG. 1 shows a liquid circulation heating system 1A according to a firstembodiment of the present invention. This liquid circulation heatingsystem 1A heats a liquid to produce a heated liquid, releases heat ofthe heated liquid from a heating radiator 3, and thereby performsair-heating, for example, in a room. Specifically, the liquidcirculation heating system 1A includes the heating radiator 3, a heatpump 2 for producing the heated liquid, and an overall controller 5 forcontrolling the entire system.

In the present embodiment, the heating radiator 3 is connected directlyto the heat pump 2 by a supply pipe 31 and a recovery pipe 32 to bedescribed later, so that the liquid flows without stopping. In theliquid circulation heating system 1A having such a configuration, thehot water produced can be used directly for air-heating. Therefore, heatradiation loss is reduced, and as a result, energy conservation can beachieved. As the liquid, for example, an antifreeze liquid containingpropylene glycol or the like dissolved in water can be used, but wateris preferably used because it is available at low cost and in largequantities. The following description will be made on the assumptionthat the liquid is water and the heated liquid is hot water.

The heat pump 2 has a heat pump circuit 20 for circulating arefrigerant. This heat pump circuit 20 includes a compressor 21 forcompressing the refrigerant, a radiator (refrigerant radiator) 22 forradiating heat from the compressed refrigerant, an expansion valve 23serving as a decompressor for decompressing the refrigerant that hasradiated heat, and an evaporator 24 for evaporating the decompressedrefrigerant. These components 21 to 24 are connected in series by pipes.The heat pump 2 includes a heat pump controller (corresponding to acontroller of the present invention) 26 for controlling the compressor21 and the expansion valve 23 according to an instruction from theoverall controller 5.

In the radiator 22, heat is exchanged between the refrigerant and thewater flowing through the radiator 22 so as to heat the water, andthereby hot water is produced. In the evaporator 24, heat is exchangedbetween the refrigerant and air blown by a fan 25, and thereby therefrigerant absorbs heat.

In the present embodiment, the heat pump circuit 20 is charged with, asa refrigerant, a zeotropic refrigerant mixture of at least tworefrigerants having different boiling points. A zeotropic refrigerantmixture means a refrigerant in which the mixture composition of a gasrefrigerant is different from that of a liquid refrigerant in avapor-liquid equilibrium state (i.e., the mixture ratio between the atleast two refrigerants in the gas refrigerant is different from that inthe liquid refrigerant). When a comparison is made between the gasrefrigerant and the liquid refrigerant, the gas refrigerant contains ahigher percentage of a low boiling point refrigerant, while the liquidrefrigerant contains a higher percentage of a high boiling pointrefrigerant. Examples of such a mixed refrigerant include R407C that isa mixture of R32, R125, and R134a, a mixture of R32 and HFO-1234yf(2,3,3,3-tetrafluoropropene), and a mixture of R32, HFO-1234yf, andR125.

It is preferable to use a zeotropic refrigerant mixture including a highboiling point refrigerant and a low boiling point refrigerant, with adifference in boiling point therebetween being 20° C. or more. Forexample, in the case of R407C containing a high boiling pointrefrigerant consisting of R32 (having a boiling point of −52° C.) andR125 (having a boiling point of −49° C.) and a low boiling pointrefrigerant that is R134a (having a boiling point of −26° C.), adifference in boiling point between these high and low boiling pointrefrigerants is 23° C.

Further, in the present embodiment, an accumulator 27 is providedbetween the evaporator 24 and the compressor 21. This accumulator 27separates the refrigerant evaporated in the evaporator 24 into a gasrefrigerant and a liquid refrigerant, and serves as the vapor-liquidseparator of the present invention. In the present embodiment, azeotropic refrigerant mixture is used. Therefore, a liquid refrigerant,which is rich in a high boiling point refrigerant, is accumulated in thebottom portion of the accumulator 27. For example, in the case where thezeotropic refrigerant mixture is a mixture of R32 (having a boilingpoint of −52° C.) and HFO-1234yf (having a boiling point of −29° C.), aliquid refrigerant, which is rich in HFO-1234yf, is accumulated in thebottom portion of the accumulator 27.

The heating radiator 3 is a device for radiating heat from hot waterflowing therethrough, and has an inlet for allowing the hot water toflow thereinto, and an outlet for allowing the hot water that hasradiated its heat to flow therefrom. As the heating radiator 3, forexample, a radiator to be placed in a room of a building may be used. Ahot water panel to be laid on a floor also may be used.

The outlet of the heating radiator 3 is connected to the radiator 22 bythe supply pipe 31 for supplying water to the radiator 22. The inlet ofthe heating radiator 3 is connected to the radiator 22 by the recoverypipe 32 for recovering hot water produced in the radiator 22. The supplypipe 31 is provided with an incoming water temperature sensor 72 fordetecting the temperature of water (hereinafter referred also as an“incoming water temperature”) to be supplied to the radiator 22. Thesupply pipe 31 also is provided with a pump 61 on the upstream side ofthe incoming water temperature sensor 72. The incoming water temperaturesensor 72 is connected to the heat pump controller 26. On the otherhand, the recovery pipe 32 is provided with a hot water temperaturesensor 71 for detecting the temperature of the hot water produced in theradiator 22. When the pump 61 is rotated, the water is guided from theheating radiator 3 to the radiator 22 by the supply pipe 31, and the hotwater produced in the radiator 22 is guided from the radiator 22 to theheating radiator 3 by the recovery pipe 32.

The overall controller 5 includes a microcomputer, a digital signalprocessor (DSP), or the like, and is connected to the above-mentionedheat pump controller 26, the hot water temperature sensor 71, and thepump 61, respectively.

Next, the control operations performed by the overall controller 5 andthe heat pump controller 26 are described specifically.

When a user turns on a heating switch (not shown), the overallcontroller 5 rotates the pump 61 and sends an operation start signal tothe heat pump controller 26. Thereby, water is heated in the radiator 22to produce hot water, and the produced hot water is fed to the heatingradiator 3. Thus, air-heating is performed.

The overall controller 5 controls the rotational rate of the pump 61 toregulate the flow rate of the water flowing through the supply pipe 31so that the temperature of the water detected by the hot watertemperature sensor 71 becomes a specified temperature (for example, 70°C.).

When the temperature of the water (incoming water temperature) suppliedto the radiator 22 is relatively high, the heat pump controller 26performs a control operation to reduce the amount of the liquidrefrigerant in the accumulator 27. Specifically, when the temperaturedetected by the incoming water temperature sensor 72 is higher than apredetermined temperature (for example, 55° C.), the heat pumpcontroller 26 reduces the opening of the expansion valve 23. When theopening of the expansion valve 23 is reduced, the refrigerant absorbsheat efficiently in the evaporator 24 and the dryness of the refrigerantflowing into the accumulator 27 increases. Thereby, the amount of theliquid refrigerant remaining in the accumulator 27 decreases. As aresult, the composition ratio of the high boiling point refrigerant inthe refrigerant circulating through the heat pump circuit 20 increases,which thereby suppresses the increase in the high pressure of therefrigeration cycle.

With reference to FIG. 3, the above-described phenomenon is explainedbelow by taking, as an example, the case where the zeotropic refrigerantmixture is a mixture of R32 and HFO-1234yf. In FIG. 3, full linesindicate the case where the refrigerant is R32 alone and the case wherethe refrigerant is HFO-1234yf alone, a single-dashed line indicates thecase where the composition ratio of HFO-1234yf in the mixed refrigerantcirculating through the heat pump circuit 20 has a certain value, and adouble-dashed line indicates the case where the composition ratio ofHFO-1234yf is higher than the certain value.

First, it is assumed that the high pressure of the refrigeration cycleis located at Point a on the single-dashed line when the incoming watertemperature is low. If the amount of the liquid refrigerant in theaccumulator 27 is fixed, the composition of the refrigerant circulatingthrough the heat pump circuit 20 does not change. Therefore, when theincoming water temperature rises to, for example, 65° C., the highpressure of the refrigeration cycle shifts from Point a to Point b alongthe single-dashed line. In the present embodiment, however, the amountof the liquid refrigerant remaining in the accumulator 27 decreases. Asa result, the composition ratio of the high boiling point refrigerant inthe refrigerant circulating through the heat pump circuit 20 increases,and thus the high pressure of the refrigeration cycle shifts from Pointa to Point c on the double-dashed line. Accordingly, the increase in thehigh pressure of the refrigeration cycle can be suppressed when theincoming water temperature rises.

That is, when the opening of the expansion valve 23 is reduced, Point Ashifts to the right in the Mollier diagram of FIG. 2 and thus thedryness increases. Thereby, the amount of the liquid refrigerantremaining in the accumulator 27 decreases. As a result, the compositionratio of the high boiling point refrigerant in the refrigerantcirculating through the heat pump circuit 20 increases, which therebysuppresses the increase in the high pressure of the refrigeration cycle.

It should be noted that the liquid refrigerant in the accumulator 27does not necessarily have to disappear completely. When the amount ofthe liquid refrigerant in the accumulator 27 decreases to some extent,the heat pump controller 26 may stop reducing the opening of theexpansion valve 23 to maintain it.

Second Embodiment

FIG. 4 shows a liquid circulation heating system 1B according to asecond embodiment of the present invention. In the present embodiment,the same components as those in the first embodiment are designated bythe same reference numerals and no further description is given.

The liquid circulation heating system 1B of the second embodiment hasbasically the same configuration as the liquid circulation heatingsystem 1A of the first embodiment, except that the heating radiator 3and the radiator 22 are connected via the hot water storage tank 8.

Furthermore, as a refrigerant charged in the heat pump circuit 20, thesame refrigerant as that described in the first embodiment also is usedin the present embodiment, and therefore the description of thezeotropic refrigerant mixture is not repeated here. The same descriptionalso is not repeated in the following embodiment and modifications.

The hot water storage tank 8 is a vertically cylindrical closed casingand is filled with water. The lower portion of the hot water storagetank 8 is connected to the radiator 22 by the supply pipe 31, and theupper portion thereof is connected to the radiator 22 by the recoverypipe 32.

When the pump 61 is rotated, the water is guided from the lower portionof the hot water storage tank 8 to the radiator 22 by the supply pipe31, and the hot water produced in the radiator 22 is guided from theradiator 22 to the upper portion of the hot water storage tank 8 by therecovery pipe 32. Thereby, the produced hot water is stored in the hotwater storage tank 8 from the top thereof. On the peripheral surface ofthe hot water storage tank 8, hot water temperature sensors 74 fordetermining how much hot water remains in the tank 8 are providedseparately from each other in the vertical direction. The hot watertemperature sensors 74 are connected to the overall controller 5.

In the present embodiment, a heat exchanger 92 for hot water supply isprovided at an upper position in the hot water storage tank 8, and awater inlet pipe 91 and a hot water outlet pipe 93 are connected to thisheat exchanger 92. That is, in the present embodiment, the produced hotwater can be used as a heat source for hot water supply. In addition, aheater 85 for re-heating the hot water also is provided at an upperposition in the hot water storage tank 8.

The inlet of the heating radiator 3 is connected to the upper portion ofthe hot water storage tank 8 by a feed pipe 81, and the outlet of theheating radiator 3 is connected to the lower portion of the hot waterstorage tank 8 by a return pipe 82. In the present embodiment, acirculation pump 66 is provided in the return pipe 82, but thecirculation pump 66 may be provided in the feed pipe 81. The circulationpump 66 is connected to the overall controller 5. When the circulationpump 66 is rotated, the hot water stored in the hot water storage tank 8is fed to the heating radiator 3 through the feed pipe 81, and the hotwater is returned to the hot water storage tank 8 through the returnpipe 82 after radiating heat in the heating radiator 3.

Next, the control operations performed by the overall controller 5 andthe heat pump controller 26 are described specifically.

(Hot Water Storage Operation)

When the overall controller 5 determines that the amount of hot waterremaining in the tank is less than the required amount based on thetemperature detected by the hot water temperature sensors 74, forexample, during nighttime hours (for example, from 23:00 to 7:00), itrotates the pump 61, and sends an operation start signal to the heatpump controller 26. Thereby, water is heated in the radiator 22 toproduce hot water, and the produced hot water is stored in the hot waterstorage tank 8. The overall controller 5 also controls the rotationalrate of the pump 61 to regulate the flow rate of the water flowingthrough the supply pipe 31 so that the temperature of the water detectedby the hot water temperature sensor 71 becomes a specified temperature(for example, 70° C.).

When the temperature of the water (incoming water temperature) suppliedto the radiator 22 is relatively high, the heat pump controller 26performs a control operation to reduce the amount of the liquidrefrigerant in the accumulator 27. Specifically, when the temperaturedetected by the incoming water temperature sensor 72 is higher than apredetermined temperature (for example, 55° C.), the heat pumpcontroller 26 reduces the opening of the expansion valve 23. When theopening of the expansion valve 23 is reduced, the refrigerant absorbsheat efficiently in the evaporator 24 and the dryness of the refrigerantflowing into the accumulator 27 increases. Thereby, the amount of theliquid refrigerant remaining in the accumulator 27 decreases. As aresult, the composition ratio of the high boiling point refrigerant inthe refrigerant circulating through the heat pump circuit 20 increases,which thereby suppresses the increase in the high pressure of therefrigeration cycle.

That is, when the opening of the expansion valve 23 is reduced, Point Ashifts to the right in the Mollier diagram of FIG. 2 and thus thedryness increases. Thereby, the amount of the liquid refrigerantremaining in the accumulator 27 decreases. As a result, the compositionratio of the high boiling point refrigerant in the refrigerantcirculating through the heat pump circuit 20 increases, which therebysuppresses the increase in the high pressure of the refrigeration cycle.

It should be noted that the liquid refrigerant in the accumulator 27does not necessarily have to disappear completely. When the amount ofthe liquid refrigerant in the accumulator 27 decreases to some extent,the heat pump controller 26 may stop reducing the opening of theexpansion valve 23 to maintain it.

(Air-Heating Operation)

When a user turns on a heating switch (not shown), the overallcontroller 5 rotates the circulation pump 66. Thereby, the hot waterstored in the hot water storage tank 8 is fed to the heating radiator 3,where heat is radiated from the hot water. Thus, air-heating isperformed.

In the liquid circulation heating system 1B of the second embodimentdescribed above, high-temperature hot water stored in the hot waterstorage tank 8 can be fed to the heating radiator 3 even during theearly stage of the air-heating operation. Therefore, air-heating can bestarted immediately after the heating switch is turned on.

Third Embodiment

FIG. 5 shows a liquid circulation heating system 1C according to a thirdembodiment of the present invention. In the present embodiment, the samecomponents as those in the first and second embodiments are designatedby the same reference numerals and no further description is given.

In the liquid circulation heating system 1C of the third embodiment, hotwater stored in the hot water storage tank 8 can be used directly forhot water supply. Specifically, the water inlet pipe 91 is connected tothe lower portion of the hot water storage tank 8, and the hot wateroutlet pipe 93 is connected to the upper portion of the hot waterstorage tank 8. At an upper position in the hot water storage tank 8, aheat exchanger 83 for exchanging heat between the hot water stored inthe hot water storage tank 8 and a heat transfer liquid (secondaryliquid) is provided. The heat exchanger 83 is connected to the heatingradiator 3 by the feed pipe 81 and the return pipe 82. When thecirculation pump 66 is rotated, the heat transfer liquid heated in theheat exchanger 83 is fed to the heating radiator 3 through the feed pipe81, and the heat transfer liquid is returned to the heat exchanger 83through the return pipe 82 after radiating heat in the heating radiator3. As the heat transfer liquid, for example, an antifreeze liquid can beused, but water preferably is used because it is available at low costand in large quantities.

Since the overall controller 5 performs control in the same manner as inthe second embodiment, the description thereof is not repeated here. Itshould be noted, however, that during the air-heating operation, theheat transfer liquid that has exchanged heat with the hot water storedin the hot water storage tank 8 radiates heat in the heating radiator 3,that is, the heat of the hot water is transferred to the heatingradiator 3 by the heat transfer liquid, and thereby air-heating isperformed.

In the liquid circulation heating system 1C having such a configuration,the temperature in the lower portion of the hot water storage tank 8 canbe kept low because of the water supplied from the water inlet pipe 91.Therefore, the low-temperature water can be supplied to the radiator 22,and thus the efficiency of the heat pump 2 can be enhanced.

(First Modification)

In each of the above embodiments, a heat pump 2A as shown in FIG. 6 alsocan be employed. In this heat pump 2A, when the temperature of the water(incoming water temperature) supplied to the radiator 22 is relativelyhigh, the refrigerant to be fed to the accumulator 27 from theevaporator 24 is heated.

Specifically, the heat pump circuit 20 includes a heat exchanger(heater) 29 between the evaporator 24 and the accumulator 27, and abypass passage 29A that bypasses the expansion valve 23 so as to passthrough the heat exchanger 29. With this configuration, the heatexchanger 29 exchanges heat between the refrigerant flowing through thebypass passage 29A and the refrigerant to be fed to the accumulator 27from the evaporator 24. The bypass passage 29 is provided with anopen/close valve 29B on the upstream side of the heat exchanger 29. Theopen/close valve 29B is connected to the heat pump controller 26, andusually is closed by the heat pump controller 26.

When the temperature detected by the incoming water temperature sensor72 is higher than a predetermined temperature, the heat pump controller26 opens the open/close valve 29B. When the open/close valve 29B isopened, the refrigerant that has been evaporated in the evaporator 24 isheated further in the heat exchanger 29, and the dryness of therefrigerant flowing into the accumulator 27 increases. Thereby, theamount of the liquid refrigerant remaining in the accumulator 27decreases. As a result, the composition ratio of the high boiling pointrefrigerant in the refrigerant circulating through the heat pump circuit20 increases, which thereby suppresses the increase in the high pressureof the refrigeration cycle.

That is, when the open/close valve 29B is opened, Point A shifts to theright in the Mollier diagram of FIG. 2 and thus the dryness increases.Thereby, the amount of the liquid refrigerant remaining in theaccumulator 27 decreases. As a result, the composition ratio of the highboiling point refrigerant in the refrigerant circulating through theheat pump circuit 20 increases, which thereby suppresses the increase inthe high pressure of the refrigeration cycle.

It should be noted that the liquid refrigerant in the accumulator 27does not necessarily have to disappear completely. When the amount ofthe liquid refrigerant in the accumulator 27 decreases to some extent,the heat pump controller 26 may stop increasing the opening of theopen/close valve 29B to maintain it.

When the incoming water temperature is relatively high, it is onlydesirable that the amount of the heating applied to the refrigerant tobe fed to the accumulator 27 from the evaporator 24 by the heatexchanger 29 increase. The open/close valve 29B does not necessarilyhave to be closed completely in its initial state.

In the first modification, the heat exchanger 29 is used as the heaterof the present invention, but the heater of the present invention is notlimited to the heat exchanger. For example, it may be an electricheater, or the like. If the heat exchanger 29 and the bypass passage 29Aare provided as in the case of the first modification, however, therefrigerant to be fed to the accumulator 27 from the evaporator 24 canbe heated by utilizing the heat of the refrigerant that has passedthrough the radiator 22.

(Second Modification)

In each of the above embodiments, a heat pump 2B as shown in FIG. 7 alsocan be employed. In this heat pump 2B, a first expansion valve 23A and asecond expansion valve 23B are used as the decompressor of the presentinvention. The refrigerant is decompressed by the first expansion valve23A after radiating heat in the radiator 22, and further decompressed bythe second expansion valve 23B after being decompressed by the firstexpansion valve 23A. A receiver 28 is provided between the firstexpansion valve 23A and the second expansion valve 23B. This receiver 28separates the refrigerant decompressed by the first expansion valve 23Ainto a gas refrigerant and a liquid refrigerant, and serves as thevapor-liquid separator of the present invention.

When the temperature detected by the incoming water temperature sensor72 is higher than a predetermined temperature, the heat pump controller26 reduces the opening of the first expansion valve 23A and increasesthe opening of the second expansion valve 23B. When the opening of thefirst expansion valve 23A is reduced and the opening of the secondexpansion valve 23B is increased, the refrigerant absorbs heatefficiently in the evaporator 24 and the dryness of the refrigerantflowing into the receiver 28 increases. Thereby, the amount of theliquid refrigerant remaining in the receiver 28 decreases. As a result,the composition ratio of the high boiling point refrigerant in therefrigerant circulating through the heat pump circuit 20 increases,which thereby suppresses the increase in the high pressure of therefrigeration cycle.

It should be noted that the liquid refrigerant in the receiver 28 doesnot necessarily have to disappear completely. When the amount of theliquid refrigerant in the receiver 28 decreases to some extent, the heatpump controller 26 may stop controlling the openings of the firstexpansion valve 23A and the second expansion valve 23B to maintain them.

(Other Modifications)

In each of the above embodiments and modifications, the heat pumpcontroller 26 serves as the controller of the present invention. In thecase where the incoming water temperature sensor 72 is connected to theoverall controller 5, the heat pump controller 26 and the overallcontroller 5 may serve as the controller of the present invention.

As the decompressor of the present invention, an expander for recoveringpower from the expanding refrigerant also can be used.

Furthermore, it is also possible, by using fraction separation, membraneseparation, or the like, to increase the composition ratio of the highboiling point refrigerant in the refrigerant circulating through theheat pump circuit 20 when the water supplied to the radiator 22 has arelatively high temperature.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this specification are to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A liquid circulation heating system for performing air-heating byheating a liquid to produce a heated liquid and releasing heat of theheated liquid from a heating radiator, the system comprising a heat pumpcircuit for circulating a refrigerant, wherein the heat pump circuitincludes a refrigerant radiator for heating the liquid by radiating heatfrom the refrigerant to produce the heated liquid, the heat pump circuitis charged with a zeotropic refrigerant mixture of at least tworefrigerants having different boiling points, as the refrigerant, andthe liquid circulation heating system is configured so that acomposition ratio of a higher boiling point refrigerant in therefrigerant circulating through the heat pump circuit increases when theliquid supplied to the refrigerant radiator has a relatively hightemperature.
 2. The liquid circulation heating system according to claim1, wherein the heat pump circuit further includes: a compressor forcompressing the refrigerant; a decompressor for decompressing therefrigerant; an evaporator for evaporating the refrigerant; and avapor-liquid separator for separating the refrigerant into a gasrefrigerant and a liquid refrigerant, and the liquid circulation heatingsystem further comprises a controller for performing a control operationto reduce an amount of the liquid refrigerant in the vapor-liquidseparator when the liquid supplied to the refrigerant radiator has arelatively high temperature.
 3. The liquid circulation heating systemaccording to claim 2, wherein the vapor-liquid separator is anaccumulator provided between the evaporator and the compressor.
 4. Theliquid circulation heating system according to claim 3, wherein thedecompressor is an expansion valve, and the controller reduces anopening of the expansion valve when the liquid supplied to therefrigerant radiator has a relatively high temperature.
 5. The liquidcirculation heating system according to claim 3, wherein the heat pumpcircuit further includes, between the evaporator and the accumulator, aheater for heating the refrigerant to be fed to the accumulator from theevaporator, and the controller increases an amount of the heatingapplied to the refrigerant by the heater when the liquid supplied to therefrigerant radiator has a relatively high temperature.
 6. The liquidcirculation heating system according to claim 5, wherein the heat pumpcircuit further includes a bypass passage that bypasses thedecompressor, and the heater is a heat exchanger for exchanging heatbetween the refrigerant flowing through the bypass passage and therefrigerant to be fed to the accumulator from the evaporator.
 7. Theliquid circulation heating system according to claim 2, wherein thedecompressor includes: a first expansion valve for decompressing therefrigerant after the refrigerant radiates heat in the refrigerantradiator; and a second expansion valve for further decompressing therefrigerant after the refrigerant is decompressed by the first expansionvalve, the vapor-liquid separator is a receiver provided between thefirst expansion valve and the second expansion valve, and the controllerreduces an opening of the first expansion valve and increases an openingof the second expansion valve when the liquid supplied to therefrigerant radiator has a relatively high temperature.
 8. The liquidcirculation heating system according to claim 1, further comprising: asupply pipe for guiding the liquid from the heating radiator to therefrigerant radiator; and a recovery pipe for guiding the heated liquidfrom the refrigerant radiator to the heating radiator.
 9. The liquidcirculation heating system according to claim 1, further comprising: atank for storing the produced heated liquid; a supply pipe for guidingthe liquid from a lower portion of the tank to the refrigerant radiator;a recovery pipe for guiding the heated liquid from the refrigerantradiator to an upper portion of the tank; a feed pipe for feeding theheated liquid stored in the tank to the heating radiator; and a returnpipe for returning the heated liquid to the tank after the heated liquidradiates heat in the heating radiator.
 10. The liquid circulationheating system according to claim 1, further comprising: a tank forstoring the produced heated liquid; a supply pipe for guiding the liquidfrom a lower portion of the tank to the refrigerant radiator; a recoverypipe for guiding the heated liquid from the refrigerant radiator to anupper portion of the tank; a heat exchanger, disposed in the tank, forexchanging heat between the heated liquid stored in the tank and a heattransfer liquid; a feed pipe for feeding the heat transfer liquid to theheating radiator after the heat transfer liquid is heated in the heatexchanger; and a return pipe for returning the heat transfer liquid tothe heat exchanger after the heat transfer liquid radiates heat in theheating radiator.
 11. The liquid circulation heating system according toclaim 1, wherein the liquid is water, and the heated liquid is hotwater.
 12. A method of controlling a liquid circulation heating systemfor performing air-heating by heating a liquid to produce a heatedliquid and releasing heat of the heated liquid from a heating radiator,wherein the liquid circulation heating system includes a heat pumpcircuit for circulating a refrigerant, and the heat pump circuitincludes: a refrigerant radiator for heating the liquid by radiatingheat from the refrigerant to produce the heated liquid; a compressor forcompressing the refrigerant; a decompressor for decompressing therefrigerant; an evaporator for evaporating the refrigerant; and avapor-liquid separator for separating the refrigerant into a gasrefrigerant and a liquid refrigerant, the heat pump circuit is chargedwith a zeotropic refrigerant mixture of at least two refrigerants havingdifferent boiling points, as the refrigerant, and the method comprisesthe step of controlling an operation of the decompressor to reduce anamount of the liquid refrigerant in the vapor-liquid separator when theliquid supplied to the refrigerant radiator has a relatively hightemperature.